Semiconductors group

Abstract:

Self-trapped excitons (STEs), which have one or both carriers spatially trapped by a lattice distortion, are associated with broadband emission and a large Stokes shift that is desirable for many applications. The fundamental physical processes that lead to their formation are difficult to observe, mainly due to the ultrafast time scales involved and the low oscillator strength of STE transitions. Here, we employ ultrafast transient absorption spectroscopy with sub-20 fs temporal resolution in the ultraviolet to study the STE formation process in a pair of lead-free double perovskites, Cs₂AgInCl₆ and Cs₂(Ag_0.6Na_0.4)InCl₆. Using first-principles calculations, we assign a broad photoinduced absorption band in Cs₂AgInCl₆ to an intraband transition in the valence band that tracks the initial 70 fs hot-hole cooling step. Furthermore, exciton-phonon coupling calculations unravel the phonon modes that couple strongly with excitons in the lowest absorption peak to cause self-trapping. The transient absorption data shows the buildup of a stimulated emission band from the STE on a 200 fs time scale and long-lived coherent oscillations corresponding to the phonons of the lattice modified by the STE formation process.

Abstract:

Alkali metals are considered as a promising alternative to conventional noble metals for plasmonic applications, offering lower optical loss and significantly reduced material costs. The recent development of a thermo-assisted spin-coating process paired with phase-shift photolithography has enabled the creation of stable nanostructured sodium, which exhibits narrow resonances in the near-infrared (NIR) region and demonstrates free electron relaxation times comparable to noble metals. Through the control of nanostructure pitch and light incident angle, the surface plasmon polariton (SPP) resonance wavelength can be tuned throughout the visible and NIR regions, making nanostructured sodium particularly attractive for nanophotonics, surface-enhanced sensing, and photocatalytic applications. In this work, we investigate hot electron dynamics in nanostructured sodium thin films on polyurethane supports by leveraging the high sensitivity of SPPs to their metal's bulk properties. Through optical transient reflectance measurements, we probe the distinct signatures of electron-electron and electron-phonon interactions in sodium at ultrafast time scales. Our results show the unique early time response of sodium that differs from those observed in noble metals, providing key insight into sodium-based plasmonics. This comprehensive understanding of hot electron dynamics will enable more efficient design and implementation of sodium in next-generation plasmonic devices and applications where hot electron processes are critical considerations.

Abstract:

Fully conjugated 3D covalent organic frameworks (COFs) are a newly emerged class of materials that expands reticular chemistry to extended electron delocalization for optoelectronic applications. To overcome the limitations of sp3‐connected 3D frameworks, the pseudo‐tetrahedral motif cyclooctatetrathiophene (COTh) has gained attention for forming fully conjugated 3D COFs. We report on a novel COTh building block, featuring functional formyl groups directly attached to the core's conjugated thiophenes. The modulation synthesis approach with mono‐functionalized inhibitors enables the formation of COTh‐1P COF, which exhibited remarkable crystallinity and permanent porosity. By following this approach and by optimizing the synthesis conditions for the solvothermal growth of thin films, we fabricated the first preferentially oriented conjugated 3D COF films on various substrates without pre‐functionalization. With these thin films, optical pump terahertz probe studies allowed us, for the first time with 3D‐fully conjugated COFs, to provide insights into the excited state and charge‐carrier dynamics of these unique organic frameworks. Low effective masses are discovered for valence and conduction bands by density functional theory simulations. The ability to create crystalline and oriented films of fully π‐conjugated 3D COTh‐based COFs on non‐modified substrates is expected to open the way for integration of such frameworks into diverse optoelectronic applications.

Abstract:

The copper–silver–bismuth–iodide compound Cu2AgBiI6 has emerged as a promising lead-free and environmentally friendly alternative to wide-bandgap lead-halide perovskites for applications in multijunction solar cells. Despite its promising optoelectronic properties, the efficiency of Cu2AgBiI6 is still severely limited by poor charge collection. Here, we investigate the impact of commonly used charge transport layers (CTLs), including poly­[bis­(4-phenyl)­(2,4,6-trimethylphenyl)­amine] (PTAA), CuI, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and SnO2, on the structural and optoelectronic properties of coevaporated Cu2AgBiI6 thin films. We reveal that while organic transport layers, such as PTAA and PCBM, form a relatively benign interface, inorganic transport layers, such as CuI and SnO2, induce the formation of unintended impurity phases within the CuI–AgI–BiI3 solid solution space, significantly influencing structural and optoelectronic properties. We demonstrate that identification of these impurity phases requires careful cross-validation combining absorption, X-ray diffraction and THz photoconductivity spectroscopy because their structural and optoelectronic properties are very similar to those of Cu2AgBiI6. Our findings highlight the critical role of CTLs in determining the structural and optoelectronic properties of coevaporated copper–silver–bismuth–iodide thin films and underscore the need for advanced interface engineering to optimize device efficiency and reproducibility.

Abstract:

The rise of metal halide perovskites has prompted a revolution in optoelectronics research [1]. Yet a comprehensive understanding of the vertical charge transport that underlies the soaring efficiencies of perovskite-based solar cells has remained challenging owing to the materials' nanocrystalline structure [2] and competing crystal phases [3]. Here, we simultaneously probe the intrinsic out-of-plane charge-carrier diffusion and the nanoscale morphology by pushing depth-sensitive terahertz near- field nano-spectroscopy [4] to extreme subcycle timescales [5]. Evanescent terahertz fields at the apex of a metallic tip probe ultrafast dynamics of photocarriers in $\text{FA}_{0}.{}_{83}\text{Cs}_{0.17}\text{Pb}(\mathrm{I}_{1}{}_{-\mathrm{x}}\mathrm{C}1_{\mathrm{x}})_{3}$ films with nanoscale spatial resolution (Fig. 1 a). By analysing characteristic phonon signatures in the spectral response, we distinguish domains of the photo active cubic $\alpha$ -phase from the trigonal δ-phase and $\text{PbI}_{2}$ nano-islands (Fig. 1 b). To examine the impact of these nanoscale inhomogeneities on carrier dynamics, we monitor the peak pump-induced signal as a function of pump delay time $t_{\mathrm{p}}$ (Fig. 1c). Notably, the full pump-induced waveforms at different $t_{\mathrm{p}}$ (Fig. 1d) show a deeply subcycle time shift $\Delta t$, which can be microscopically linked to a diffusion-driven change in the vertical carrier distribution with $t_{\mathrm{r}}$ (Fig. 1e, inset). Com-bining a straightforward rate equation model with the finite-dipole model, we extract the evolution of the out-of-plane carrier density profile from $\Delta t$ (Fig. 1e). Mapping the out-of-plane diffusion coefficient $D$ along a line across different grains, including $\alpha$ -phase grains and $\text{PbI}_{2}$ contaminations, we find a homogeneous value of $D=(0.2\pm 0.1)\ \text{cm}^{2}\mathrm{s}^{-1}$, which is surprisingly immune to compositional and structural variations (Fig. 1 f). This robustness of vertical diffusion may contribute to the exceptional performance of perovskite-based devices. Linking in situ carrier transport with nanoscale morphology and chemical composition could introduce a paradigm shift for the analysis and optimization of next-generation optoelectronics that are based on nanocrystalline materials.